1. Field of the Disclosure
The present disclosure relates to a semiconductor laser device that converts a wavelength of light emitted by a semiconductor laser. In particular, the present disclosure relates to a semiconductor device used, for example, in an optical recording apparatus, a measuring instrument, or a business machine.
2. Description of the Related Art
In recent years, because of features such as small size, high efficiency, and high directivity, a semiconductor laser has been used in a wide range of applications including an optical recording apparatus, a measuring instrument, a printer, a medical device, and a business machine. In particular, using a semiconductor laser in a laser pointer, which requires small size and high directivity, is well known. Japanese Registered Utility Model No. 3123345 proposes a laser pointer that uses laser light having a wavelength range with relative luminosity higher than that of red light generally used. FIG. 6 illustrates a laser pointer 800 in which an optical resonator 807 includes a solid laser medium 803 and a nonlinear optical element 804. The solid laser medium 803 is excited by light emitted from a semiconductor laser 801. The nonlinear optical element 804 converts a wavelength of light generated by excitation of the solid laser medium 803. With this configuration, laser light is output, which has a wavelength shorter than that of red light.
In the technique of wavelength conversion realized by the semiconductor laser 801, the solid laser medium 803, and the nonlinear optical element 804, the luminous efficiency depends largely on the temperature. This means that self-heating and the external environment may cause unstable laser output. Therefore, as illustrated in FIG. 6, the laser pointer 800 proposed by Japanese Registered Utility Model No. 3123345 includes an automatic power control (APC) circuit 811 for stable light output. The APC circuit 811 has a feedback mechanism in which output light from the optical resonator 807 is partially reflected by a beam splitter 806 and is incident on a detector 810, so that the output of light can be stabilized on the basis of output from the detector 810.
In monitoring of output light for the APC function according to related art, emitted light that leaks backward is monitored in an edge-emitting semiconductor laser (see, e.g., Japanese Unexamined Patent Application Publication No. 2008-275505), whereas diffuse reflected light reflected back from a cover glass on a laser aperture is monitored in a surface-emitting semiconductor laser where there is no backward leakage of emitted light (see, e.g., Japanese Unexamined Patent Application Publication No. 2007-185850). However, when light emitted from the semiconductor laser is used as a fundamental wave and wavelength-converted by a transducer, since the temperature dependency of luminous efficiency with the transducer is not uniform, output light cannot be accurately controlled by monitoring the light emitted from the semiconductor laser. Therefore, in the technique of Japanese Registered Utility Model No. 3123345, light converted by a transducer is monitored.
Japanese Unexamined Patent Application Publication No. 2004-281932 proposes a method which does not involve use of a beam splitter. FIG. 7 illustrates a laser emitting module 900 in which an optical resonator 956 includes a solid laser medium 905 and a nonlinear optical element 906. The solid laser medium 905 is excited by light emitted from a laser diode 904. The nonlinear optical element 906 converts a wavelength of light generated by excitation of the solid laser medium 905. With this configuration, laser light is output, which has a wavelength shorter than that of red light.
Monitoring of output light for the APC function is performed by a process in which output light from the optical resonator 956 is partially specularly reflected by an optical filter 907 on a laser aperture, further reflected by a reflecting mirror 909 held by a window cap 953, passed through an optical filter 908, and is incident on a photodiode 911 mounted on a header 901. Thus, from the light emitted by the optical resonator 956, a fundamental wave from the laser diode 904 and a wave excited by the solid laser medium 905 are eliminated by the optical filter 907 and the optical filter 908. This means that the light that reaches the photodiode 911 contains only a desired converted wave. Therefore, with the APC function, it is possible to accurately control the laser diode 904, efficiently emit a desired converted wave from a window 903, and stabilize the output of the converted wave.
In the related art disclosed in Japanese Registered Utility Model No. 3123345, output light from the optical resonator 807 is directly split by the beam splitter 806 and is incident on the detector 810. In this technique, the incident output light contains not only a desired converted wave, but also a fundamental wave of the semiconductor laser 801 and a wave exited by the solid laser medium 803. Therefore, these undesired waves affect the detector 810 as noise and send a false signal to the APC function. As a solution to this, a filter that eliminates such undesired waves may be added between the optical resonator 807 and the beam splitter 806. However, this not only increases the number of components and costs, but also hinders size reduction.
In the related art disclosed in Japanese Unexamined Patent Application Publication No. 2004-281932, an optical path of output light from the optical resonator 956 is altered and the light is passed through the optical filter 907 and the optical filter 908, so that only a desired converted wave is incident on the photodiode 911. However, altering the optical path of output light requires the window cap 953 of complex shape. Therefore, joining the window cap 953 to a flange 952 of a heat sink requires adjustment of the optical axis which involves both horizontal and rotational positioning. At the same time, using the additional optical filter 908 for complete elimination of the undesired waves and the reflecting mirror 909 for backward reflection results in an increased number of components and increased costs.
The present invention has been made to solve the problems described above. The present invention provides a semiconductor laser device which is compact, allows easy adjustment of the optical axis, and requires no additional components for monitoring output light for the APC function.